To conduct a production test of an offshore well from a floating vessel, it has become conventional practice to employ a subsea master valve apparatus, or "test tree", that includes a relatively short-length valve body which is releasably latched to a hydraulically operable control unit. The apparatus is positioned and landed in a subsea blowout preventer stack that is located on the sea floor. The master valve section is connected to a production string of pipe that extends downwardly into a well, and the control unit is connected to a length of production pipe that extends upwardly within a riser to the floating vessel. Examples of related devices of this general type are shown in U.S. Pat. Nos. 3,967,647 and 3,955,623, assigned to the assignee of this invention. These devices have the significant advantage over other systems, shown for example in U.S. Pat. No. RE 27,464, that the hydraulic control unit can be disconnected from the valve body in case of an emergency, and removed with the pipe from the riser, leaving the valve body in place. The valve body houses upper and lower valve elements to provide redundant shut-in valves, with the upper valve being a normally closed flapper element and the lower valve being a normally closed ball element. Both valves are opened in response to pressurization of a control line that leads to the control unit, and are automatically closed whenever the control unit is unlatched and released from the valve body. The valve functions to shut-off the productioon string against upward flow of well fluids. The relatively short length of the valve body enables blind rams of the blowout preventer stack to be closed against one another at a location above the upper end of the valve body to ensure complete control of the well, and there are no exposed hydraulic connections or the like which would otherwise be subject to contamination by sea water or drilling fluids.
It also is well known to provide a "retainer" valve in the production string above the control unit. This valve, which is a normally open device, functions when closed in response to the pressure applied to a control line at the surface, to accelerate the disconnection of the control unit and valve body by suppressing the requirement to bleed of pressure in the upper production pipe. The retainer also enables pressure testing of the upper pipe for leaks prior to well testing. The retainer valve is of particular importance when testing in deep water because it can be closed to prevent pollution and dumping of high pressure gases within the riser.
The use of a separately positioned retainer valve has in the past required the presence of separate sets of hydraulic control lines for operating the master valves and the retainer valve. This requirement has resulted in complex hydraulic systems which are somewhat complicated for an operator to manipulate. Moreover, since the retainer valve has been positioned well above the control unit, a volume of well fluids and/or gases could still be dumped in the interior of the riser when the control unit is disconnected. In accordance with the present invention, the retainer valve system is an integral part of the control unit and is actuated in response to hydraulic pressures applies thereto. This construction greatly simplifies the overall arrangement of the tool and its operational sequence.
Another shortcoming of prior devices results from the fact that it is possible to trap fluids under pressure in the region between the shut-in valve and the retainer valve, which is undesirable for various reasons. One feature of the present invention lies in the provision of a bleed valve to vent pressure from this region, with the bleed valve being opened in response to a control pressure which causes release of the latch, and which opens prior to disconnection. The bleed valve cannot be opened so long as there is a control pressure being applied to maintain the shut-in valve open, and it is designed for fail-safe closure in response to well pressure. In case of hydraulic failure, the bleed valve can be opened mechanically when actuating an emergency mechanical means to disconnect the latch.
Another feature of the present invention is the provision of a ball valve element that closes fail-safe under the influence of both a spring and the pressure of a yieldable medium such as nitrogen gas. Closing also is assisted by well pressure, and the hydraulic pressure applied to a control line. The ball valve is opened, and held open, by hydraulic pressure applied to a separate control line, and is designed such that if opening pressure is lost, closure of the ball valve is provoked.
Still another feature of the new and improved subsea master valve of the present invention is the use of passages and ports within various structural members such that any fluid that leaks past a dynamic seal that is exposed to well pressure (a seal that moves relatively along another member when the tool is operated) will cause the system to assume a safe condition with the shut-in ball element closed. This feature provides additional assurance that the well will at all times be under control.
A further unique feature of the present invention is the provision of a flapper element in the valve body that is automatically pushed open when the control unit is connected, and which is held open by nitrogen and spring pressure. The flapper valve can be closed in response to hydraulic pressure applied to a control line, and will automatically close when the control unit is disconnected from the valve body.
It is the general object of the present invention to provide a new and improved subsea master valve apparatus having an integral retainer valve as well as the other unique features specified above.